feat: add usage cost reporting

docs/tools/skills.md

@@ -163,6 +163,23 @@ This is **scoped to the agent run**, not a global shell environment.
 
 Clawdbot snapshots the eligible skills **when a session starts** and reuses that list for subsequent turns in the same session. Changes to skills or config take effect on the next new session.
 
+## Token impact (skills list)
+
+When skills are eligible, Clawdbot injects a compact XML list of available skills into the system prompt (via `formatSkillsForPrompt` in `pi-coding-agent`). The cost is deterministic:
+
+- **Base overhead (only when ≥1 skill):** 195 characters.
+- **Per skill:** 97 characters + the length of the XML-escaped `<name>`, `<description>`, and `<location>` values.
+
+Formula (characters):
+
+```
+total = 195 + Σ (97 + len(name_escaped) + len(description_escaped) + len(location_escaped))
+```
+
+Notes:
+- XML escaping expands `& < > " '` into entities (`&amp;`, `&lt;`, etc.), increasing length.
+- Token counts vary by model tokenizer. A rough OpenAI-style estimate is ~4 chars/token, so **97 chars ≈ 24 tokens** per skill plus your actual field lengths.
+
 ## Managed skills lifecycle
 
 Clawdbot ships a baseline set of skills as **bundled skills** as part of the